The bi-directional optical transceiver commonly used in passive optical network (PON) and fiber-to-the-home (FTTH) systems are mainly of two types, duplexer and triplexer. The duplexer processes both data and voice signals, while the triplexer processes data, voice and video signals. As the increasing demands of digital and analog video transmission, conventional duplexer cannot meet the market requirements, and triplexer becomes the trend of optical communication in the future.
A duplexer optical subassembly mainly includes a light emitter and a light receiver. The light emitter is mainly a laser diode; the light receiver is mainly a photo diode. Whatever an emitter or a receiver, the optical path requires alignment. In the emitter side, there are alignment problems between the laser diode and the single-mode fiber or planar waveguide. Because the alignment tolerance of a single-mode fiber or planar waveguide is just 1 or 2 microns, it is easy to be misaligned and poor coupling, and the power output and quality of transmission is turned down. Generally, there are two manners to improve alignment. The first is an active alignment of lighting the laser diode and aligning through coupling. It can achieve precise alignment, however it costs much. The second is a passive alignment of applying an alignment key for assembly. It greatly saves assembly cost, however the process is difficult to be approached.
In a triplex optical transceiver, at the light emitter side, the laser beam from the laser diode has to pass through several splitters and filters before getting into a single-mode fiber. Because the laser beam transmitting in a free space diffuses in accordance with its transmission distance, it encounters a problem that the final laser beam coupled to the single-mode fiber is less. In order to solve the problem, micro lenses have to be used for increasing the numerical aperture. However, using micro lens increases the cost and the complication of assembly.
For a duplex optical transceiver, there are commonly planar waveguides or tubular waveguides available. A planar waveguide optical subassembly includes three major optical coupling interfaces: the laser diode with a planar waveguide, the planar waveguide with another planar waveguide via a filter, and the planar waveguide with a single-mode fiber. The three interfaces all encounter problems of optical misalignment. Different types of waveguide further have problems of field mismatch. Therefore, it is hard to improve the coupling efficiency of the whole unit. Tubular waveguide optical subassembly mainly uses lens to solve the problem of free space optical misalignment. The alignment tolerance is compensated by lens. However, the coupling efficiency is still low and the lens increases the cost.
Only tubular optical subassembly is used in a triplex optical transceiver. The tubular light emitter and tubular light receiver increase the cost of the optical components. Further, the tubular components have larger dimensions that increase the coupling length in free space and cause lower efficiency and light dispersion in the transmission.
As described above, whatever for a planar waveguide optical subassembly or a tubular optical subassembly, the cost of the assembly in the coupling interface is hard to be reduced.